1. Technical Field
The present invention is directed to the art of welding a stud to a workpiece and, more particularly, to improvements in controlling the welding to compensate for variations in arc length as a result of workpiece variations or stud length variations.
2. Prior Art
Welding tools for welding studs, sometimes referred to herein as fasteners, to a metal workpiece are known in the art. Typically, a welding tool caries a stud, which is resiliently urged to an extended position. The stud is retracted slightly to a work position when the stud is pressed against a workpiece to place it in good electrical contact therewith. A trigger is then actuated to initiate a welding operation. The stud is then withdrawn from the workpiece toward a fully retracted position and a pilot arc is established. Thereafter, when the stud is in its fully retracted position, a main welding arc is established between the stud and the workpiece. Then the stud is plunged toward the workpiece and, upon making electrical contact therewith, the welding arc is extinguished. Welding tools that perform the above functions are described in my previous U.S. Pat. Nos. 3,291,958; 3,136,880 and 3,392,257.
It has been determined that the arcing time in stud welding will vary due to changes in arc length caused by variations in workpiece or stud length. Unless compensation is provided, there will be a tendency to have inconsistent weld quality from weld to weld. In my previous U.S. Pat. No. 3,392,257, provisions are made for sensing the position of a stud to be welded prior to the beginning of a weld cycle. If a stud is not in a proper position, the welding circuit is rendered inoperative until a proper stud position is attained. No provisions are made for controlling the plunging of the stud toward the workpiece as a function of measured displacement in order to control the point in time at which the stud makes contact with the workpiece.